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nfs_localio: use cmpxchg() to install new nfs_file_localio

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Rather than using nfs_uuid.lock to protect installing
a new ro_file or rw_file, change to use cmpxchg().
Removing the file already uses xchg() so this improves symmetry
and also makes the code a little simpler.

Also remove the optimisation of not taking the lock, and not removing
the nfs_file_localio from the linked list, when both ->ro_file and
->rw_file are already NULL.  Given that ->nfs_uuid was not NULL, it is
extremely unlikely that neither ->ro_file or ->rw_file is NULL so
this optimisation can be of little value and it complicates
understanding of the code - why can the list_del_init() be skipped?

Finally, move the assignment of NULL to ->nfs_uuid until after
the last action on the nfs_file_localio (the list_del_init).  As soon as
this is NULL a racing nfs_close_local_fh() can bypass all the locking
and go on to free the nfs_file_localio, so we must be certain to be
finished with it first.

Fixes: 86e00412254a ("nfs: cache all open LOCALIO nfsd_file(s) in client")
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>
This commit is contained in:
NeilBrown 2025-05-09 10:46:38 +10:00 committed by Anna Schumaker
parent 111f9e4b0d
commit ed9be31733
2 changed files with 20 additions and 30 deletions
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11
fs/nfs/localio.c
View File

@ -282,14 +282,9 @@ nfs_local_open_fh(struct nfs_client *clp, const struct cred *cred,
return NULL;
rcu_read_lock();
/* try to swap in the pointer */
spin_lock(&clp->cl_uuid.lock);
nf = rcu_dereference_protected(*pnf, 1);
if (!nf) {
nf = new;
new = NULL;
rcu_assign_pointer(*pnf, nf);
}
spin_unlock(&clp->cl_uuid.lock);
nf = unrcu_pointer(cmpxchg(pnf, NULL, RCU_INITIALIZER(new)));
if (!nf)
swap(nf, new);
}
nf = nfs_local_file_get(nf);
rcu_read_unlock();

39
fs/nfs_common/nfslocalio.c
View File

@ -273,8 +273,8 @@ EXPORT_SYMBOL_GPL(nfs_open_local_fh);
void nfs_close_local_fh(struct nfs_file_localio *nfl)
{
struct nfsd_file *ro_nf = NULL;
struct nfsd_file *rw_nf = NULL;
struct nfsd_file *ro_nf;
struct nfsd_file *rw_nf;
nfs_uuid_t *nfs_uuid;
rcu_read_lock();
@ -285,28 +285,23 @@ void nfs_close_local_fh(struct nfs_file_localio *nfl)
return;
}
ro_nf = rcu_access_pointer(nfl->ro_file);
rw_nf = rcu_access_pointer(nfl->rw_file);
if (ro_nf || rw_nf) {
spin_lock(&nfs_uuid->lock);
if (ro_nf)
ro_nf = rcu_dereference_protected(xchg(&nfl->ro_file, NULL), 1);
if (rw_nf)
rw_nf = rcu_dereference_protected(xchg(&nfl->rw_file, NULL), 1);
ro_nf = unrcu_pointer(xchg(&nfl->ro_file, NULL));
rw_nf = unrcu_pointer(xchg(&nfl->rw_file, NULL));
/* Remove nfl from nfs_uuid->files list */
RCU_INIT_POINTER(nfl->nfs_uuid, NULL);
list_del_init(&nfl->list);
spin_unlock(&nfs_uuid->lock);
rcu_read_unlock();
if (ro_nf)
nfs_to_nfsd_file_put_local(ro_nf);
if (rw_nf)
nfs_to_nfsd_file_put_local(rw_nf);
return;
}
spin_lock(&nfs_uuid->lock);
/* Remove nfl from nfs_uuid->files list */
list_del_init(&nfl->list);
spin_unlock(&nfs_uuid->lock);
rcu_read_unlock();
/* Now we can allow racing nfs_close_local_fh() to
* skip the locking.
*/
RCU_INIT_POINTER(nfl->nfs_uuid, NULL);
if (ro_nf)
nfs_to_nfsd_file_put_local(ro_nf);
if (rw_nf)
nfs_to_nfsd_file_put_local(rw_nf);
}
EXPORT_SYMBOL_GPL(nfs_close_local_fh);